Clear MediaController::m_scriptExecutionContext when the ScriptExecutionContext is destroyed

HTMLMediaElement is an ActiveDOMObject, so it's possible that HTMLMediaElement
outlives ScriptExecutionContext. Hence the HTMLMediaElement has to clear out
the ScriptExecutionContext of MediaController that the HTMLMediaElement owns.

Review URL: https://codereview.chromium.org/25989002

git-svn-id: svn://svn.chromium.org/blink/trunk@158967 bbb929c8-8fbe-4397-9dbb-9b2b20218538